The present invention generally relates to a hydraulic play compensating element for a valve train of an internal-combustion engine, and more particularly to a play compensation element adapted to be rendered inoperative as a function of the operating parameters of the internal combustion engine.
Hydraulic play compensating elements are known, such as disclosed by DE-PS 2,606,464, wherein an accidental readjustment of the compensating element is intended to be prevented by throttle gaps and inlet ducts in the oil circuit of the element upstream of the supply chamber.
However, it is impossible to interrupt the readjustment of the above-noted hydraulic play compensating element in a controlled way in response to specific operating conditions of the internal-combustion engine. Engine braking service is one such operating condition, for example. The exhaust gases which accumulate in the exhaust pipe upstream of the closed exhaust flap generate a counterforce on the valve springs through the outlet valves, which can become so great that a brief dynamic opening of the outlet valves can occur. The compensating element within the valve train is relieved by this means and readjusts. This has the disadvantage that the outlet valves no longer close in service under load.
Accordingly, it is an object of the invention to provide a hydraulic play compensating element in which the readjustment function can be interrupted in a controlled way.
This object is achieved according to the present invention by providing a hydraulic play compensating element comprising a cylinder part closed at one end, a hollow piston part slidable axially in the cylinder part and having a supply chamber adapted to be pressurized by lubricating oil pressure of the internal combustion engine. A pressure chamber, enclosed by the hollow piston part and the cylinder part is included for providing play compensation for the valve train of the internal combustion engine. A compression spring is also included for biasing the cylinder part and the piston part in opposite directions. A connecting duct connects the pressure chamber and the supply chamber, such that a spring biased closure element for opening and closing the connecting duct, operates independently of a further controlled shut-off valve for opening and closing the connecting duct of the closure element.
Due to the provision of the further controlled shut-off valve in the connecting duct, the readjustment function of the play compensating element can be interrupted arbitrarily in an advantageous manner.
Because an interruption of the readjustment function is generally desired only in specific operating modes of the internal-combustion engine, it is convenient if, according to the one embodiment of the present invention, the shut-off valve of the present invention is controlled as a function of operating parameters of the internal-combustion engine.
In accordance with advantageous features of preferred embodiments of the present invention, the lubricating oil from the lubricating oil circuit, which is provided as pressurized fluid for the play compensation, ensures a simple mode of control of the shut-off valve when the operation of the shut-off valve is actuated by the lubricating oil as a function of the oil pressure.
In order to obtain a compensating element of simple construction, it is advantageous, according to certain preferred embodiments of the present invention, to arrange the shut-off valve in the supply chamber of the piston part.
A simple structural configuration is obtained according to certain preferred embodiments of the present invention by constructing the shut-off valve as a stepped piston axially slidable at least in a direction counter to a return spring positioned at a wall of the piston part opposite the connecting duct and providing the stepped piston with a closure element which cooperates with a valve seat surface positioned in a region of the orifice of the connecting duct in the supply chamber.
In that a reliable interruption of the readjustment function of the play compensation element may be prejudiced by an overflow of leakage oil from the pressure chamber to the supply chamber or vice versa, through the ring gap between piston part and cylinder part, for example, in accordance with preferred embodiments of the present invention the ring gap is made particularly small, or alternatively it is bridged by a ring seal.
According to further preferred embodiments of the present invention, a capillary overflow bore is provided between pressure chamber and supply chamber to compensate for thermal expansion of the lubricating oil cushion in the pressure chamber and thermal longitudinal expansions of the valve train.
A discharge of oil from the supply chamber into the pressure chamber is possible through the overflow bore. The minimal readjustment movement of the piston part caused by this is undesirable when the readjustment function is switched off. For these reasons the orifice of the overflow bore is arranged, in accordance with certain preferred embodiments of the present invention, in the region of the valve seat surface of the shut-off valve. When the readjustment function is switched off, the shut-off valve closes the connecting duct and the overflow bore simultaneously.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.